Grounding switch method and apparatus

Abstract

A grounding switch is described which operates properly even in the presence of negative voltages on a signal line. The grounding switch uses isolated field effect transistors that have their substrates tied to different voltages. The isolated field effect transistor has a gate voltage and substrate voltage which can be pulled down to a negative voltage when the signal line has a negative voltage allowing the switch to remain open even with a negative voltage.

What is claimed: 
     
       1. A method of suppressing an audible pop during power up of an audio driver having an output driver operable to drive an output signal coupled to a first transistor in series with a second transistor, the method comprising:
 turning on the first transistor and the second transistor during power up; 
 turning off the first transistor after power up and the output signal has a positive voltage; and 
 turning off the second transistor after power up and the output signal has a negative voltage, wherein the second transistor includes a gate and an isolated substrate and turning off the second transistor comprises pulling down the gate to a voltage maintained by the isolated substrate. 
 
     
     
       2. The method of  claim 1  wherein turning on the second transistor comprises pulling up the gate to the positive supply voltage. 
     
     
       3. The method of  claim 1  wherein:
 turning off the second transistor further comprises pulling down the gate to a negative supply voltage; and 
 maintaining the isolated substrate at the negative supply voltage. 
 
     
     
       4. The method of  claim 1  wherein:
 turning off the second transistor further comprises pulling down the gate to the voltage of the signal line; and 
 maintaining the isolated substrate at the voltage of the signal line. 
 
     
     
       5. The method of  claim 1  wherein turning on the first transistor and the second transistor during power up comprises:
 receiving a control signal; and 
 turning on the first transistor and the second transistor when the control signal is high. 
 
     
     
       6. The method of  claim 1  wherein turning off the first transistor after power up and the output signal has a positive voltage comprises:
 receiving a control signal; and 
 turning off the first transistor when the control signal is low. 
 
     
     
       7. The method of  claim 1  wherein turning off the second transistor after power up and the output signal has a negative voltage comprises:
 receiving a control signal; and 
 turning off the second transistor when the control signal is low. 
 
     
     
       8. The method of  claim 1  wherein turning on the first transistor causes the second transistor to turn on. 
     
     
       9. A grounding switch coupled to a signal line having a voltage comprising:
 means for turning on a first transistor when a control input is high; 
 means for turning on a second transistor when the control input is high; 
 means for turning off the first transistor when the control input is low and the voltage of the signal line is positive; and 
 means for turning off the second transistor when the control input is low and the voltage of the signal line is negative. 
 
     
     
       10. The grounding switch of  claim 9  wherein the second transistor comprises a gate and the means for turning on the second transistor comprises means for pulling up the gate to the positive supply voltage. 
     
     
       11. The grounding switch of  claim 9  wherein the second transistor comprises a gate and a substrate and the means for turning off the second transistor comprises pulling down the gate to a negative supply voltage; and means for maintaining the substrate at a negative supply voltage. 
     
     
       12. The grounding switch of  claim 9  wherein the second transistor comprises a gate and a substrate and the means for turning off the second transistor comprises means for pulling down the gate to the voltage of the signal line; and means for maintaining the substrate at the voltage of the signal line. 
     
     
       13. A method of suppressing an audible pop during power up of an audio driver comprising:
 turning on a first transistor and a second transistor of a grounding switch during a power up interval; 
 turning off the first transistor after the power up interval and when an output signal has a positive voltage; and 
 turning off the second transistor after the power up interval and when the output signal has a negative voltage, wherein the second transistor includes a gate and an isolated substrate and turning off the second transistor comprises pulling down the gate to a voltage maintained by the isolated substrate. 
 
     
     
       14. The method of  claim 13  wherein turning on the second transistor comprises pulling up the gate to the positive supply voltage. 
     
     
       15. The method of  claim 13  wherein:
 the turning off the second transistor comprises pulling down the gate to a negative supply voltage; and 
 maintaining the isolated substrate at a negative supply voltage. 
 
     
     
       16. The method of  claim 13  wherein:
 the turning off the second transistor comprises pulling down the gate to the voltage of the signal line; and 
 maintaining the isolated substrate at the voltage of the signal line. 
 
     
     
       17. The method of  claim 13  wherein:
 the second transistor comprises a gate and the turning on the second transistor comprises pulling up the gate to the positive supply voltage; 
 the second transistor comprises a gate and an isolated substrate and the turning off the second transistor comprises pulling down the gate to a negative supply voltage; and 
 maintaining the isolated substrate at a negative supply voltage. 
 
     
     
       18. The method of  claim 13  wherein:
 the second transistor comprises a gate and the turning on the second transistor comprises pulling up the gate to the positive supply voltage; 
 the second transistor comprises a gate and an isolated substrate and the turning off the second transistor comprises pulling down the gate to the voltage of the signal line; and 
 maintaining the isolated substrate at the voltage of the signal line. 
 
     
     
       19. The method of  claim 13  wherein turning on the first transistor causes the second transistor to turn on. 
     
     
       20. A method of suppressing an audible pop during power up of an audio driver comprising:
 receiving a control signal; 
 turning on a first transistor and a second transistor of a grounding switch during a power up interval when the control signal is high; 
 turning off the first transistor after the power up interval and when an output signal has a positive voltage when the control signal is low; and 
 turning off the second transistor after the power up interval and when the output signal has a negative voltage and the control signal is low, wherein the second transistor includes a gate and an isolated substrate and turning off the second transistor comprises pulling down the gate to a voltage maintained by the isolated substrate. 
 
     
     
       21. The method of  claim 20  wherein turning on the first transistor causes the second transistor to turn on.